If I get red blocks in the error test does that mean the data in that red block is 100% unreadable and gone/corrupt or can it still be read but at a slower speed?

A red block means that a read error occurred even after multiple retries. Changing read speed may have some effect on its readability, its however very doubtful as a slow down occurs when a read error is found anyway.

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Cheers NicW

Rudyard Kipling wrote: If you can keep your head when all about you are losing theirs and blaming it on you....If you can bear to hear the truth you've spoken twisted by knaves to make a trap for fools

Using a different drive is a better way of changing your results, and possibly saving your data. Of course, if your disc is this badly damaged, it would be a good idea to make a copy of it, if another drive is capable of it.